1. Field of the Invention
The invention relates to a device and a method for UV irradiation, particularly for disinfection, of flowing liquids with reduced UV transmission. The device has one or more support frames with a plurality of UV modules, each of which contains a low-pressure mercury-vapour radiator. The UV modules are kept apart in substantially parallel manner by the support frames and extend in the direction of flow of the liquid, which is treated waste water in particular.
2. Summary of the Prior Art
Devices of this kind are used in particular in sewage treatment plants because nowadays the treated waste water from treatment plants increasingly frequently needs to be disinfected before being introduced into natural watercourses. For this purpose the waste water treated by the treatment plant is exposed to ultraviolet radiation (UV irradiation). The UV modules with low-pressure mercury-vapour radiators used for this generate ultraviolet rays in the wavelength range from approx. 200 nm to 300 nm with a maximum at 253.7 nm.
The UV modules are in the form of immersion UV radiators and each comprise a low-pressure mercury-vapour radiator which is surrounded by a sheathing tube permeable to UV radiation. Corresponding devices of the type described thus far for UV irradiation are known from documents EP 0687201, DE 3270676, U.S. Pat. Nos. 4,757,205, 5,019,256, 4,482,809, EP 0080780 and EP 0249450 for example.
In contrast to clean, unpolluted water, treated waste water which is to be disinfected exhibits greatly reduced UV transmission. It is normally in the range of 40% to 60% per 1 cm of layer thickness. This means that 40% -60% of the UV radiation applied is absorbed in a water layer 1 cm thick and the effective depth of penetration is restricted to only approx. 5 cm (by comparison, pure drinking water has a transmission in the range of approx. 90% to 98%, so that the absorption losses are only 2% to 10% per cm of layer thickness, which corresponds to a comparable effective depth of penetration of approx. 2 m to 2.5 m).
The consequence of poor UV transmission and low effective depth of penetration for UV radiation is that such media can only be irradiated effectively in relatively thin layer thicknesses, such as are found, for example, in the immediate circumferential vicinity of radiator sheathing tubes (UV modules).
In such applications, the UV dose required to kill micro-organisms can only be applied with the traditional devices if the medium is in contact with the UV radiation for a fairly time, i.e. at low flow rates, because they are equipped with UV modules which have small cross-sections in relation to the irradiation chambers surrounding them on the one hand and have UV radiator sources with low radiation output on the other hand.
The traditional devices are equipped with UV modules which are approx. 1 to 1.5 m long, the low-pressure mercury-vapour radiators of which, with diameters of 15 to 25 mm maximum, generate UV emissions in output values of 0.18 to 0.66 W/cm of discharge length and the sheathing tubes of which do not exceed an external diameter of 33 mm maximum, the annular gap which is formed between the external diameter of the radiator tube and the internal diameter of the sheathing tube having a width of 2.5 mm maximum. The full radiation output of 0.66 W/cm of discharge length of the UV modules which are the most powerful and technically most advanced at the present time requires temperatures of the surrounding medium of more than 20xc2x0 C. because of the low insulating effect of the small annular gap. Radiation outputs of 0.5 W/cm or 0.4 W/cm can only be achieved when medium temperatures of 15 to 20xc2x0 C. and/or greater than 10xc2x0 C. are maintained. Furthermore, the distances between the UV modules and hence the irradiation chambers which result are dimensioned in such a way that their volumes are approx. 12 to 16 times as large or larger than the volume of the UV modules (including UV radiators). Particularly high and reliable disinfection performances, i.e. disinfection performances which are not subject to substantial variations under the conventional transmission changes, cannot be guaranteed in practice with these ratios of variables.
The object of the present invention is therefore to render possible for such liquids a device for more effective, more reliable disinfection effects with shorter contact times, i.e. for larger quantities of waste water per unit of time at higher and turbulent flow rates.
Because there is assigned to each UV module an irradiation chamber of polygonal cross-section, the midpoint of which is the midpoint of the UV radiator at the same time and which, starting from the midpoint, extends as far as half the distance to the adjacent UV modules, the individual UV modules being arranged in such a way that the midpoints of a number of adjacent UV modules form the corner points of a polygon, the area of which corresponds to the sum of the cross-sectional area of a UV module and the cross-sectional area of the irradiation chamber of a UV module, and that the distance between the UV modules is selected in such a way that the cross-sectional area of the irradiation chamber of each UV module is not more than 10 times larger than the cross-sectional area of the UV module itself, the high energy density may be utilized preferentially to shorten the contact times, i.e. to increase the flow rates and/or throughputs.
Furthermore, practical experiments have surprisingly shown that with the same dimensions, the enlargement alone of the UV module diameter and the associated reduction of the irradiation chambers again leads to an increase and above all a stabilization of the disinfection effect. When disinfecting waste water it is of the utmost importance that no substantial variations in the disinfection effect take place during the typical changes in the UV transmission which are conventional in this case. Compliance with the following dimensions has proved to be particularly advantageous according to experience gained from extensive practical experiments:
Maximum ratio of the volumes of the UV modules to the volumes of the irradiation chambers at UV transmissions up to T(1 cm)=40%:
Annular gap width R as a function of the temperature of the medium:
0xc2x0 C. to 10xc2x0 C. R greater than 4 mm
10xc2x0 C. to 25xc2x0 C. R greater than 3 mm
UV radiator tube diameters and UV radiation outputs:
Ø15 to 20 mm-0.3 W/cm
Ø25 mm up to 0.6 W/cm
Ø32 mm up to 1.1 W/cm
Ø38 mm up to 1.3 W/cm.
In pursuance of this objective the UV modules are equipped with high-performance low-pressure mercury-vapour radiators which with radiator tube diameters in the range from 22 to 36 mm have radiation outputs of approx. 0.6 W to more than 1.1 W/cm of discharge quantity in the spectral range from 200 nm to 300 nm and hence generate greater irradiances at the surfaces of the sheathing tubes than have been possible hitherto.
In practical experiments it has surprisingly been shown that a higher disinfection effect can be achieved by an enlargement of the sheathing tube diameter and the accompanying reduction of the irradiation chambers.
The overall outcome to emerge from these measures is that in order to achieve at least equivalent or higher disinfection effects, the volumes of the irradiation chambers can be reduced from the currently conventional 12 to 18 times the volumes of the UV modules to 5 to 11 times and particularly to 7.5 to 10 times.
The distance between the UV modules is selected in such a way that the cross-section of the irradiation chamber which is produced for each UV module is not more than 16.5 times larger than the cross-section of the UV module for module diameters of 28 mm, not more than 14 times larger for module diameters of 45 mm and not more than 11.5 times larger for module diameters of 55 mm, and that the low-pressure mercury-vapour radiators of the UV modules have a radiation output of at least 0.55 W/cm of discharge length in the spectral range of 200-300 nm and that irradiances of at least 60 mW/cm2 for 28 mm, at least 50 mW/cm2 for 35 mm, at least 38 mW/cm2 for 45 mm and at least 30 mW/cm2 for 55 mm result.
Preferably the cross-section of the irradiation chamber of a UV module is not more than 7.5 times larger than the cross-section of the UV module itself.
The object of the present invention is also to provide a method for UV irradiation, particularly for disinfection, of flowing liquids with reduced UV transmission in which an increased flow rate of the liquid is possible.
This is achieved in that the distance between the UV modules is selected in such a way that the cross-section of the irradiation chamber which is produced for each UV module at a module diameter of at least 28 mm is dimensioned in such a way that the average flow rate is greater than 0.25 m/s and the radiation output of the modules in the new state is at least 0.55 W UV per centimetre of discharge length.
The term high-performance low-pressure mercury radiator refers to radiators with a discharge length of at least 70 cm and a radiation output of more than 0.55 W per cm in the new state and at least 0.2 W/cm at the end of the service life and/or as the smallest control value. The term xe2x80x9cirradiation chamberxe2x80x9d, which is produced for each UV module, will be explained in greater detail below.